Fix completion for Debian Apt aliases

The current Debian plugin uses single quotes for Apt commands to enable switching between `apt-get` and `aptitude`. Unfortunately, this disables completion. By substituting the `$apt_pref` variable during alias definition instead of execution, completion of Apt commands is enabled again.

Note that this does mean that the `$apt_pref` variable needs to be set *before* loading `oh-my-zsh` instead of after.